Zoom lens system for a lens shutter camera

ABSTRACT

A zoom lens system is composed of two lens units. The first lens unit is arranged at an object-side end and the second lens unit is arranged at an image-side end. The first lens unit has one lens element of a positive refractive power. The second lens unit has two lens elements. One is an aspherical plastic lens of non-refractive power. The other is a lens having a negative refractive power. During zooming from a wide angle end to a telephoto end, the distance between the first and second lens units is decreased.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No. 08/712,257, filed on Sep. 11, 1996, now U.S. Pat. No. 5,815,324.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system, more particularly to a zoom lens system suitably used as a taking lens system in lens shutter cameras.

2. Description of the Prior Art

It is required that a zoom lens system used as a taking lens system in lens shutter cameras be made as compact and low-cost as possible, without diminishing their optical performance. As such lens system, various zoom lens systems that consist of a first lens unit having a positive refractive power and a second lens unit having a negative refractive power have been proposed.

Zoom lens systems consisting of two lens groups, however, are defective in that it is not possible, especially at the wide-angle end, to properly correct magnification chromatic aberration and distortion at the same time as securing sufficient lens-backs required in a taking lens system for cameras.

Recently, even in the zoom lens system consisting of two lens groups, use of single lenses made of plastic is preferred nowadays in consideration of cost reduction. It is not possible, however, to obtain sufficient optical performance by simply aking such lens-system-constituting single lenses using lastic. This is because ambient temperature variation causes ariation in the shape and refractive power of the plastic lenses, with the result that the focal point of the whole system varies. To solve this problem, U.S. Pat. No. 5,368,321 and Japanese Laid-Open Patent Application No. H5-113537 propose a two-lens-group zoom lens system in which the variation in focal point of a plastic lens is corrected. This type of zoom lens system, however, is defective in that the plastic lens in it is far from serving well enough to correct aberration, because the plastic lens is designed to have almost no refractive power.

Moreover, U.S. Pat. No. 5,327,290 proposes a zoom lens system consisting of a front lens unit composed of one positive lens, and a rear lens unit composed of one negative lens.

Moreover, U.S. Pat. No. 5,218,478 proposes a zoom lens system consisting of a first lens unit composed of two single lenses and having a positive refractive power as a whole, and a second lens unit composed of one negative lens.

The zoom lens systems disclosed in U.S. Pat. Nos. 5,327,290 and 5,218,478, however, are defective in that chromatic aberration cannot be corrected properly.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a zoom lens system that offers sufficient optical performance despite its compactness.

Another object of the present invention is to provide a zoom lens system whose image forming performance is less susceptible to ambient temperature variation.

Still another object of the present invention is to provide a zoom lens system in which chromatic aberration can be corrected properly despite its compactness.

To achieve the above objects, according to the present invention, a zoom lens system comprises, from the object-side end, a first lens unit having a positive refractive power, and a second lens unit having a negative refractive power, wherein the distance between the first and second lens units is decreased during zooming from the shortest focal length end to the longest focal length end.

Alternatively, according to the present invention, in a zoom lens system comprising, from the object-side end, a first lens unit having a positive refractive power, and a second lens unit having a negative refractive power, wherein the distance between the first and second lens units is decreased during zooming from the shortest focal length end to the longest focal length end, said first and second lens units each include at least one plastic lens, and the following conditions are satisfied:

    -0.6<φLp1/φ1<-0.2

    0.2<φLp2/φ1<0.6

where

φLp1: refractive power of the plastic lens in the first lens unit;

φLp2: refractive power of the plastic lens in the second lens unit;

φ1: composite refractive power of the first unit.

Alternatively, according to the present invention, in a zoom lens system comprising, from the object-side end, a first lens unit consisting of one positive lens element, and a second lens unit consisting of two lens elements and having a negative refractive power as a whole, the distance between the first and second lens units is decreased during zooming from the wide-angle end to the telephoto end.

According to another aspect of the present invention, in a zoom lens system comprising, from the object-side end, a first lens unit consisting of one positive lens element, and a second lens unit having a negative refractive power, the distance between the first and second lens units is decreased during zooming from the wide-angle end to the telephoto end, and the following condition is satisfied:

    0.9<|φ1/φ2|<1.3

where

φ1: composite refractive power of the first lens unit;

φ2: composite refractive power of the second lens unit.

Alternatively, according to the present invention, in a zoom lens system comprising, from the object side, a first lens unit composed of two single lens elements and having a positive refractive power as a whole, and a second lens unit composed of one negative lens element, the distance between the first and second lens units is decreased during zooming from the shortest focal length end to the longest focal length end, and the following conditions are satisfied:

    N21<1.55

    ν21>55

where

N21: d-lines refractive coefficient of the negative lens in the second lens unit;

ν21: Abbe number of the negative lens in the second lens unit.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of this invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which:

FIG. 1 is a diagram showing the lens construction of a first embodiment of the present invention;

FIG. 2 is a diagram showing the lens construction of a second embodiment of the present invention;

FIG. 3 is a diagram showing the lens construction of a third embodiment of the present invention;

FIGS. 4A to 4I are diagrams showing aberration curves of the first embodiment;

FIGS. 5A to 5I are diagrams showing aberration curves of the second embodiment;

FIGS. 6A to 6I are diagrams showing aberration curves of the third embodiment;

FIG. 7 is a diagram showing the lens construction at the wide-angle end of a fourth embodiment of the present invention;

FIG. 8 is a diagram showing the lens construction at the wide-angle end of a fifth embodiment of the present invention;

FIG. 9 is a diagram showing the lens construction at the wide-angle end of a sixth embodiment of the present invention;

FIG. 10 is a diagram showing the lens construction at the wide-angle end of a seventh embodiment of the present invention;

FIG. 11 is a diagram showing the lens construction at the wide-angle end of a eighth embodiment of the present invention;

FIGS. 12A to 12I are diagrams showing aberration curves of the fourth embodiment;

FIGS. 13A to 13I are diagrams showing aberration curves of the fifth embodiment;

FIGS. 14A to 14I are diagrams showing aberration curves of the sixth embodiment;

FIGS. 15A to 15I are diagrams showing aberration curves of the seventh embodiment;

FIGS. 16A to 16I are diagrams showing aberration curves of the eighth embodiment;

FIG. 17 is a diagram showing the lens construction of a ninth embodiment of the present invention;

FIG. 18 is a diagram showing the lens construction of a tenth embodiment of the present invention;

FIG. 19 is a diagram showing the lens construction of a eleventh embodiment of the present invention;

FIGS. 20A to 20I are diagrams showing aberration curves of the ninth embodiment;

FIGS. 21A to 21I are diagrams showing aberration curves of the tenth embodiment;

FIGS. 22A to 22I are diagrams showing aberration curves of the eleventh embodiment;

FIG. 23 is a diagram showing the lens construction of a twelfth embodiment of the present invention;

FIG. 24 is a diagram showing the lens construction of a thirteenth embodiment of the present invention;

FIG. 25 is a diagram showing the lens construction of a fourteenth embodiment of the present invention;

FIG. 26 is a diagram showing the lens construction of a fifteenth embodiment of the present invention;

FIGS. 27A to 27I are diagrams showing aberration curves of the twelfth embodiment;

FIGS. 28A to 28I are diagrams showing aberration curves of the thirteenth embodiment;

FIGS. 29A to 29I are diagrams showing aberration curves of the fourteenth embodiment; and

FIGS. 30A to 30I are diagrams showing aberration curves of the fifteenth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, zoom lens systems embodying the present invention will be presented. Note that, in each embodiment, ri (r=1, 2, 3, . . . ) represents the curvature radius of the i-th surface from the object-side end, di (i=1, 2, 3, . . . ) represents the i-th axial distance from the object-side end, and Ni (i=1, 2, 3, . . . ) and νi (i=1, 2, 3, . . . ) represent the d-lines refractive coefficient and the Abbe number, respectively, of the i-th lens from the object-side end. Moreover, values of the axial distance d7 between the first and second lens units, the focal length f of the whole system, and the f-number Fno are shown; these values correspond, from the left, to the wide-angle end (W), the medium focal length (M), and the telephoto end (T). Furthermore, the surface marked with ∞ (infinite) in its curvature radius column is a surface that is perpendicular to the optical axis (that is, the curvature radius of that surface is infinite).

Note also that, in each embodiment, a surface marked with * in its curvature radius column is an aspherical surface, the shape of which can be defined by the following expression (A). ##EQU1## where X: displacement from the reference surface in the optical axis direction;

Y: height in the direction perpendicular to the optical axis;

C: reference curvature of the aspherical surface;

ε: quadric surface parameter;

Ai (i=1, 2, 3 . . . ): aspherical coefficient;

Yi (i=1, 2, 3 . . . ): Y raised to the i-th power.

Furthermore, note that the digits following the letter E in a value of aspherical coefficient represent the exponent portion of the aspherical coefficient; for example, 1.0×10E2 represents 1.0×10².

                  TABLE 1                                                          ______________________________________                                         Embodiment 1                                                                   f = 35.7˜46.0˜68.6                                                 Fno = 4.98˜6.42˜9.57                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1*   22.648                                                                                  d1     1.250 N1  1.58340 ν1                                                                              30.23                              r2    21.358                                                                                  d2     0.711                                                    r3    95.401                                                                                  d3     1.600 N2  1.84666 ν2                                                                              23.83                              r4    33.289                                                                                  d4     3.700                                                    r5    58.259                                                                                  d5     2.017 N3  1.51680 ν3                                                                              64.20                              r6   -10.460                                                                                  d6     2.050                                                    r7   ∞ (aperture)                                                                 d7   8.851˜5.676˜2.050                                    r8*  -22.375                                                                                  d8     2.000 N4  1.58340 ν4                                                                              30.23                              r9   -20.727                                                                                  d9     4.881                                                    r10   -7.681                                                                                  d10    1.000 N5  1.65844 ν5                                                                              50.85                              r11  -17.944                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r1                                                                                         ε =  0.10000 × 10                                                A4 =        -0.21745 × 10E-3                                             A6 =        -0.16445 × 10E-5                                             A8 =        -0.50958 × 10E-7                                             A10 =       -0.57513 × 10E-8                                             A12 =        0.10815 × 10E-9                                 r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.18231 × 10E-3                                             A6 =        -0.38461 × 10E-5                                             A8 =         0.26085 × 10E-6                                             A10 =       -0.53713 × 10E-8                                             A12 =        0.45517 × 10E-10                                ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                         Embodiment 2                                                                   f = 35.7˜46.0˜68.6                                                 Fno = 4.98˜6.42˜9.57                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1*   19.096                                                                                  d1     1.250 N1  1.58340 ν1                                                                              30.23                              r2    18.946                                                                                  d2     0.600                                                    r3    17.549                                                                                  d3     1.600 N2  1.84666 ν2                                                                              23.83                              r4    11.888                                                                                  d4     5.000                                                    r5    37.729                                                                                  d5     2.067 N3  1.51680 ν3                                                                              64.20                              r6   -10.589                                                                                  d6     2.050                                                    r7   ∞ (aperture)                                                                 d7   8.069˜5.259˜2.050                                    r8*  -25.103                                                                                  d8     2.000 N4  1.58340 ν4                                                                              30.23                              r9   -26.998                                                                                  d9     4.928                                                    r10   -7.032                                                                                  d10    1.000 N5  1.65844 ν5                                                                              50.85                              r11  -14.699                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r1                                                                                         ε =  0.10000 × 10                                                A4 =        -0.17216 × 10E-3                                             A6 =        -0.19014 × 10E-5                                             A8 =         0.56135 × 10E-7                                             A10 =       -0.30319 × 10E-8                                             A12 =        0.39430 × 10E-10                                r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.23048 × 10E-3                                             A6 =        -0.56151 × 10E-5                                             A8 =         0.41478 × 10E-6                                             A10 =       -0.97333 × 10E-8                                             A12 =        0.96996 × 10E-10                                ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Embodiment 3                                                                   f = 35.7˜46.0˜68.6                                                 Fno = 4.99˜6.42˜9.58                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1*   18.065                                                                                  d1     1.250 N1  1.58340 ν1                                                                              30.23                              r2    13.740                                                                                  d2     0.770                                                    r3    67.818                                                                                  d3     2.000 N2  1.84666 ν2                                                                              23.83                              r4    37.147                                                                                  d4     3.930                                                    r5    58.259                                                                                  d5     2.221 N3  1.51680 ν3                                                                              64.20                              r6   -10.709                                                                                  d6     1.350                                                    r7   ∞ (aperture)                                                                 d7   9.605˜6.403˜2.747                                    r8*  -24.009                                                                                  d8     2.966 N4  1.58340 ν4                                                                              30.23                              r9   -18.317                                                                                  d9     4.034                                                    r10   -8.031                                                                                  d10    1.000 N5  1.79950 ν5                                                                              42.34                              r11  -17.981                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r1                                                                                         ε =  0.10000 × 10                                                A4 =        -0.20601 × 10E-3                                             A6 =        -0.74300 × 10E-6                                             A8 =        -0.55943 × 10E-7                                             A10 =       -0.50266 × 10E-9                                             A12 =        0.21981 × 10E-10                                r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.17143 × 10E-3                                             A6 =        -0.29827 × 10E-5                                             A8 =         0.19737 × 10E-6                                             A10 =       -0.37945 × 10E-8                                             A12 =        0.30673 × 10E-10                                ______________________________________                                    

FIGS. 1 to 3 show the lens constructions corresponding to fthe embodiments 1 to 3, respectively, each showing the lens arrangement at the wide-angle end (W). In FIGS. 1 to 3, the arrows m1 and m2 schematically indicate the movement of the first and second lens units Gr1 and Gr2, respectively, from the wide-angle end (W) to the telephoto end (T).

The zoom lens systems of the embodiments 1 to 3 comprise, from the object-side end, a first lens unit Gr1 composed of a meniscus-shaped first lens L1 with its convex surface facing toward the object side (with its aspherical surface facing toward the object side), a meniscus-shaped second lens L2 having a negative refractive power with its convex surface facing toward the object side, a biconvex third lens having a positive refractive power, and an aperture diaphragm A, and a second lens unit Gr2 composed of a meniscus-shaped fourth lens having a positive refractive power with its convex surface facing toward the image side (with its aspherical surface facing toward the object side), and a meniscus-shaped fifth lens having a negative refractive power with its concave surface facing toward the object side. Of these lenses, the first lens L1 is a negative meniscus-shaped lens in the embodiments 1 and 3, and is a positive meniscus-shaped lens in the embodiment 2. In each embodiment, the first lens L1 and the fourth lens L4 are plastic lenses.

In the zoom lens systems of the embodiments 1 to 3, the first lens unit Gr1 is composed of, from the object-side end, a first lens L1 that does not have a strong refractive effect on the light rays passing through it, a second lens L2 having a negative refractive power, and a third lens having a positive refractive power. Owing to such construction, the arrangement of refractive powers over the whole system becomes analogous to that of a retrofocus lens, making available a sufficient lens-back at the wide-angle end. Consequently, the outer diameter of the second lens unit Gr2 can be made smaller, and the whole system can be made more compact. In addition, if a negative lens is used as the first lens having a weak refractive power, the construction becomes more analogous to the arrangement for a retrofocus lens; this is effective in securing a sufficient lens-back at the wide-angle end.

Moreover, the zoom lens systems of the embodiments 1 to 3 satisfy the following conditional expression (1):

    0.15<φ1×T23<0.25                                 (1)

where

φ1: composite refractive power of the first lens unit;

T23: air gap between the second and third lenses.

The conditional expression (1) defines a condition that needs to be satisfied by the air gap between the second and third lenses L2 and L3 in the first lens unit Gr1, and is related to correction of magnification chromatic aberration and distortion. When the lower limit of the conditional expression (1) is exceeded, the negative and positive lenses are too close to each other, and accordingly it is difficult to correct distortion. Reversely, when the upper limit of the conditional expression (1) is exceeded, the refractive power of the negative lens in the first lens unit is so strong that magnification chromatic aberration, in particular, cannot be corrected easily.

Moreover, the zoom lens systems of the embodiments 1 to 3 use as the first lens L1 of the first lens unit Gr1 a plastic lens with an aspherical surface facing toward the object side. In general, in a lens that is disposed nearest to the object side, a light ray incident on a given point on the lens surface varies its path to a greater degree according as the angle of view varies than in a lens that is disposed, for example, close to the aperture diaphragm. In addition, this light path depends on the surface shape of the lens disposed nearest to the object side. For this reason, by varying the surface shape of the lens disposed nearest to the object side, the light path can be varied greatly. Accordingly, if at least one aspherical surface is employed within the first lens L1, spherical aberration and off-axial coma aberration can be corrected effectively. Moreover, if a plastic lens is used as the aspherical first lens L1, costs can be reduced, compared to an aspherical lens made of glass.

Furthermore, the zoom lens systems of the embodiments 1 to 3 satisfy the following conditional expressions (2) and (3):

    -0.4<fT×φL1<0.4                                  (2)

    -2.5<φL1×fT.sup.2 ×(βT-βW)/FnT+φL4×fT.sup.2 /FnT<2.5(3)

where

φL1: refractive power of the first lens L1;

fT: focal length of the whole system at the telephoto end (T);

βT: magnification of the second lens unit at the telephoto end (T);

βW: magnification of the second lens unit at the wide-angle end (W);

FnT: f-number at the telephoto end (T);

φL4: refractive power of the fourth lens L4.

The conditional expressions (2) and (3) define conditions that need to be satisfied when plastic lenses are used as the first lens L1 in the first lens unit Gr1 and as the fourth lens L4 in the second lens unit Gr2. These conditional expressions are related to the variation in focus position of the whole system in accordance with the temperature variation.

The conditional expression (2) defines the refractive power of the first lens L1, which is a plastic lens in the first lens unit Gr1. When the upper limit of the conditional expression (2) is exceeded, the refractive power of the first lens in the first lens unit Gr1 varies with temperature variation, with the result that the focal point of the whole system shifts greatly toward the image side, destabilizing the optical performance. Reversely, when the lower limit is exceeded, in a similar way, a variation in the refractive power of the first lens L1 causes the focal point of the whole system to shift greatly toward the object side, destabilizing the optical performance.

The conditional expression (3) defines the relations between the various parameters of the plastic lens L4 in the second lens unit Gr2 and the refractive power. When the upper limit of the conditional expression (3) is exceeded, the refractive power of the fourth lens L4 in the second lens unit Gr2 varies with temperature variation, with the result that the focus position of the whole system shifts greatly toward the image side, destabilizing the optical performance. Reversely, when the lower limit is exceeded, in a similar way, variation in the refractive power of the fourth lens L4 causes the focus position of the whole system to shift greatly toward the object side, destabilizing the optical performance.

The zoom lens systems of the embodiments 1 to 3 of the present invention satisfy the conditional expressions (1) to (3) above. In Table L4 below are shown the values corresponding to the conditional expressions (1) to (3) in each embodiment. Note that, in the table, T23 corresponds to d4.

                                      TABLE 4                                      __________________________________________________________________________                             Conditional                                                    Conditional                                                                            Conditional                                                                            Expression (3)                                                 Expression (1)                                                                         Expression (2)                                                                         φ1 × fT.sup.2 × (βT -                                     βW)/Fnt                                                   φ1 × T23                                                                     fT × φ1                                                                      + φ4 × fT.sup.2 /Fnt                         __________________________________________________________________________     Embodiment 1                                                                           0.162   -0.069  0.77                                                   Embodiment 2                                                                           0.228   0.034   -0.12                                                  Embodiment 3                                                                           0.174   -0.624  -2.02                                                  __________________________________________________________________________

FIGS. 4A to 6I are diagrams showing aberration curves corresponding to the embodiments 1 to 3, respectively. FIGS. 4A to 4C, 5A to 5C, and 6A to 6C show aberrations at the wide-angle-end focal length (at the shortest focal length), FIGS. 4D to 4F, 5D to 5F, and 6D to 6F show aberrations at the middle focal length, and FIGS. 4G to 4I, 5G to 5I, and 6G to 6I show aberrations at the telephoto-end focal length (at the longest focal length). Moreover, solid lines (d) show aberrations with d-lines, and broken lines (SC) show sine conditions. Furthermore, broken lines (DM) and solid lines (DS) show astigmatisms on the meridional image surface and on the sagittal image surface, respectively.

Hereinafter, the embodiments L4 to 8 of the present invention will be presented on Tables 5 to 9. Note that Tables 5 to 9 are described in the same manner as Tables 1 to 3.

                  TABLE 5                                                          ______________________________________                                         Embodiment 4                                                                   f = 35.7˜46.1˜68.7                                                 Fno = 5.14˜6.62˜9.88                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1   -56.647                                                                                  d1     1.250 N1  1.83350 ν1                                                                              21.00                              r2   -86.832                                                                                  d2     0.600                                                    r3*   18.312                                                                                  d3     2.474 N2  1.58340 ν2                                                                              30.23                              r4    12.934                                                                                  d4     4.969                                                    r5    45.051                                                                                  d5     2.858 N3  1.48749 ν3                                                                              70.44                              r6   -10.814                                                                                  d6     2.051                                                    r7   ∞ (aperture)                                                                 d7   9.071˜5.806˜2.077                                    r8*  -25.517                                                                                  d8     2.405 N4  1.58340 ν4                                                                              30.23                              r9   -17.825                                                                                  d9     4.595                                                    r10   -9.101                                                                                  d10    1.000 N5  1.74400 ν5                                                                              44.93                              r11  -31.046                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r3                                                                                         ε =  0.10000 × 10                                                A4 =        -0.15829 × 10E-3                                             A6 =        -0.14263 × 10E-5                                             A8 =        -0.18713 × 10E-7                                             A10 =       -0.11403 × 10E-9                                             A12 =        0.30998 × 10E-11                                r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.73390 × 10E-4                                             A6 =         0.22545 × 10E-5                                             A8 =         9.10174 × 10E-6                                             A10 =       -0.25522 × 10E-8                                             A12 =        0.20689 × 10E-10                                ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                         Embodiment 5                                                                   f = 35.5˜46.0˜66.0                                                 Fno = 5.05˜6.55˜9.39                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1   -33.376                                                                                  d1     1.250 N1  1.83350 ν1                                                                              21.00                              r2   -33.944                                                                                  d2     0.600                                                    r3*   22.943                                                                                  d3     2.329 N2  1.58340 ν2                                                                              30.23                              r4    15.004                                                                                  d4     5.000                                                    r5    54.517                                                                                  d5     3.000 N3  1.49310 ν3                                                                              83.58                              r6   -11.220                                                                                  d6     2.051                                                    r7   ∞ (aperture)                                                                 d7   9.202˜5.651˜2.012                                    r8*  -22.257                                                                                  d8     3.000 N4  1.58340 ν4                                                                              30.23                              r9   -16.456                                                                                  d9     3.786                                                    r10   -8.172                                                                                  d10    1.000 N5  1.74400 ν5                                                                              44.90                              r11  -20.205                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r3                                                                                         ε =  0.10000 × 10                                                A4 =        -0.15254 × 10E-3                                             A6 =        -0.17824 × 10E-5                                             A8 =         0.70671 × 10E-7                                             A10 =       -0.36621 × 10E-8                                             A12 =        0.53340 × 10E-10                                r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.15626 × 10E-3                                             A6 =        -0.22856 × 10E-5                                             A8 =         0.16229 × 10E-6                                             A10 =       -0.29959 × 10E-8                                             A12 =        0.23499 × 10E-10                                ______________________________________                                    

                  TABLE 7                                                          ______________________________________                                         Embodiment 6                                                                   f = 35.7˜46.0˜68.6                                                 Fno = 4.99˜6.42˜9.58                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1*   17.377                                                                                  d1     1.600 N1  1.58340 ν1                                                                              30.23                              r2    12.367                                                                                  d2     6.000                                                    r3    57.013                                                                                  d3     3.000 N2  1.49310 ν2                                                                              83.58                              r4   -11.228                                                                                  d4     2.051                                                    r5   ∞ (aperture)                                                                 d5   9.204˜5.751˜2.000                                    r6*  -22.653                                                                                  d6     3.000 N3  1.58340 ν3                                                                              30.23                              r7   -16.648                                                                                  d7     3.953                                                    r8    -8.309                                                                                  d8     1.000 N4  1.74400 ν4                                                                              44.90                              r9   -21.354                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r1                                                                                         ε =  0.10000 × 10                                                A4 =        -0.15730 × 10E-3                                             A6 =        -0.49544 × 10E-6                                             A8 =        -0.40290 × 10E-7                                             A10 =        0.18305 × 10E-9                                             A12 =        0.20862 × 10E-11                                r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.14685 × 10E-3                                             A6 =        -0.23810 × 10E-5                                             A8 =         0.15384 × 10E-6                                             A10 =       -0.27695 × 10E-8                                             A12 =        0.21005 × 10E-10                                ______________________________________                                    

                  TABLE 8                                                          ______________________________________                                         Embodiment 7                                                                   f = 35.7˜46.0˜66.9                                                 Fno = 5.04˜6.50˜9.45                                               Curvature  Axial Surface                                                                             Refractive                                               Radius     Distance   Index         Abbe Number                                ______________________________________                                         r1*   26.709                                                                                  d1     2.016 N1  1.58340 ν1                                                                              30.23                              r2    14.987                                                                                  d2     4.500                                                    r3    57.457                                                                                  d3     4.331 N2  1.48749 ν2                                                                              70.44                              r4   -10.977                                                                                  d4     2.051                                                    r5   ∞ (aperture)                                                                 d5   11.521˜7.413˜2.960                                   r6*  -25.622                                                                                  d6     3.000 N3  1.58340 ν3                                                                              30.23                              r7   -15.175                                                                                  d7     3.250                                                    r8    -8.999                                                                                  d8     1.000 N4  1.70154 ν4                                                                              41.15                              r9   -33.232                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                         r1                                                                                         ε =  0.10000 × 10                                                A4 =        -0.16777 × 10E-3                                             A6 =        -0.32082 × 10E-6                                             A8 =        -0.48004 × 10E-7                                             A10 =        0.65184 × 10E-9                                             A12 =       -0.26099 × 10E-11                                r8                                                                                         ε =  0.10000 × 10                                                A4 =         0.10856 × 10E-3                                             A6 =        -0.14254 × 10E-5                                             A8 =         0.78421 × 10E-7                                             A10 =       -0.11772 × 10E-8                                             A12 =        0.78719 × 10E-11                                ______________________________________                                    

                  TABLE 9                                                          ______________________________________                                         Embodiment 8                                                                   ______________________________________                                         f = 34.8˜44.9˜64.0                                                 Fno = 5.05˜6.55˜9.39                                               Curvature                                                                               Axial Surface Refractive                                              Radius   Distance      Index      Abbe Number                                  ______________________________________                                         r1 -34.500                                                                              d1 1.250      N1 1.83350 ν1 21.00                                  r2 -34.490                                                                              d2 0.600                                                              r3* 22.943                                                                              d3 2.329      N2 1.58340 ν2 30.23                                  r4 15.004                                                                               d4 5.000                                                              r5 54.517                                                                               d5 3.000      N3 1.49310 ν3 83.58                                  r6 -11.220                                                                              d6 2.051                                                              r7 ∞(aperture)                                                                    d7 9.202˜5.651˜2.012                                      r8* -22.257                                                                             d8 3.000      N4 1.58340 ν4 30.23                                  r9 -16.456                                                                              d9 3.786                                                              r10 -8.172                                                                              d10 1.000     N5 1.74400 ν5 44.90                                  r11 -20.205                                                                    ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r3                                                                             ε = 0.10000 × 10                                                 A4 = -0.15254 × 10E-3                                                    A6 = -0.17824 × 10E-5                                                    A8 = 0.70671 × 10E-7                                                     A10 = -0.36621 × 10E-8                                                   A12 = 0.53340 × 10E-10                                                   r8                                                                             ε = 0.10000 × 10                                                 A4 = 0.15626 × 10E-3                                                     A6 = -0.22856 × 10E-5                                                    A8 = 0.16229 × 10E-6                                                     A10 = -0.29959 × 10E-8                                                   A12 = 0.23499 × 10E-10                                         ______________________________________                                    

FIGS. 7 to 11 show the lens constructions corresponding to the embodiments 4 to 8, respectively, each showing the lens arrangement at the wide-angle end (W). In FIGS. 7 to 11 , the arrows m1 and m2 schematically indicate the movement of the first and second lens units Gr1 and Gr2, respectively, from the wide-angle end (W) to the telephoto end (T).

The zoom lens systems of the embodiments4 to 8 comprise, from the object-side end, a first lens unit Gr1 having a positive refractive power and a second lens unit Gr2 having a negative refractive power.

In the embodiments 4, 5, and 8, the first lens unit Gr1 comprises, from the object-side end, a meniscus-shaped first lens L1 with its concave surface facing toward the object side, a negative meniscus-shaped second lens L2 with its convex surface facing toward the object side (with its aspherical surface facing toward the object side), a biconvex third lens L3 having a positive refractive power, and an aperture diaphragm A. Of these lenses, the first lens L1 has a negative refractive power in the embodiment 4, has a weak negative refractive power in the embodiment 5, and has a weak positive refractive power in the embodiment 8.

In the embodiment 4, 5, and 8, the second lens unit Gr2 comprises, from the object-side end, a positive meniscus-shaped fourth lens L4 with its convex surface facing toward the object-side (with its aspherical surface facing toward the object side), and a negative meniscus-shaped fifth lens L5 with its concave surface facing toward the object side.

In the zoom lens systems of the embodiments 4, 5, and 8, out of the above-mentioned lenses, the second lens L2 and the fourth lens L4 are plastic lenses.

In the embodiments 6 and 7, the first lens unit Gr1 comprises, from the object-side end, a negative meniscus-shaped first lens L1 with its convex surface facing toward the object side (with its aspherical surface facing toward the object side), a biconvex second lens L2 having a positive refractive power, and an aperture diaphragm A.

In the embodiment 6 and 7, the second lens unit Gr2 comprises, from the object-side end, a positive meniscus-shaped third lens L3 with its convex surface facing toward the object-side (with its aspherical surface facing toward the object side), and a negative meniscus-shaped fourth lens L4 with its concave surface facing toward the object side.

In the zoom lens systems of the embodiments 6 and 7, out of the above-mentioned lenses, the first lens L1 and the third lens L3 are plastic lenses.

In Table 10 below are shown the values of refractive power of each single lens in the embodiments 4 to 8. As seen from the table, the absolute values of the first lens L1 in the embodiments 5 and 8 are less than approximately one tenth of the absolute values of the second to fifth lenses L2 to L5 in the embodiments 5 and 8.

                  TABLE 10                                                         ______________________________________                                         Embodi-      Embodi-  Embodi-  Embodi-                                                                               Embodi-                                  ment 4       ment 5   ment 6   ment 7 ment 8                                   ______________________________________                                         1st Lens                                                                               -0.0050  -0.0000  -0.0120                                                                               -0.0160                                                                               0.0004                                 2nd Lens                                                                               -0.0110  -0.0120  0.0518 0.0518 -0.0120                                3rd Lens                                                                               0.0550   0.0522   0.0110 0.0173 0.0522                                 4th Lens                                                                               0.0110   0.0110   -0.0529                                                                               -0.0559                                                                               0.0110                                 5th Lens                                                                               -0.0567  -0.0523  --     --     -0.0523                                ______________________________________                                    

The zoom lens systems of the embodiments4 to 8 of the present invention satisfy the conditional expressions (4) to (7), which are described later. In Table 11 below are shown the values corresponding to the conditional expressions (4) to (7) in each embodiment. Note that, in Table 11, T corresponds to d4 for the embodiments 4, 5, and 8, and corresponds to d2 for the embodiments 6 and 7.

                  TABLE 11                                                         ______________________________________                                                Condi- Condi-                Condi-                                            tional tional   Conditional Ex-                                                                             tional                                            Expres-                                                                               Expres-  pression (6) Expres-                                           sion   sion     φLp1 × fT.sup.2 ×                                                           sion                                              (4)    (5)      (βT - βW)/FnT +                                                                   (7)                                               φLp1/φ1                                                                       φLp2/φ1                                                                         φLp2 × fT.sup.2 /FnT                                                              φ1 × T                           ______________________________________                                         Embodiment 4                                                                            -0.26    0.26     -2.16      0.212                                    Embodiment 5                                                                            -0.28    0.26     -2.22      0.215                                    Embodiment 6                                                                            -0.28    0.26     -2.47      0.257                                    Embodiment 7                                                                            -0.40    0.43     -1.30      0.180                                    Embodiment 8                                                                            -0.28    0.25     -1.82      0.217                                    ______________________________________                                    

The conditional expressions will be described below.

    -0.6<φLp1/φ1<-0.2                                  (4)

    0.2<φLp2/φ1<0.6                                    (5)

    -2.5<φLp1×fT.sup.2 ×(βT-βW)/FnT+φLp2×fT.sup.2 /FnT<2.5(6)

where

φLp1: refractive power of the plastic lens in the first lens unit;

φLp2: refractive power of the plastic lens in the second lens unit;

φ1: composite refractive power of the first lens unit;

fT: focal length of the whole system at the telephoto end;

βT: magnification of the second lens unit at the telephoto end;

βW: magnification of the second lens unit at the wide-angle end;

FnT: f-number at the telephoto end.

The conditional expression (4) to (6) show conditions that need to be satisfied by a two-lens-group zoom lens system in which each lens unit includes at least one plastic lens. By finding values satisfying these conditional expressions simultaneously, the refractive power of a plastic lens can be determined properly. This makes it possible to correct aberration properly and to reduce variation in focal point of the whole zoom lens system due to temperature variation, thereby achieving stable image forming performance.

The conditional expression (4) defines the refractive power of the plastic lens in the first lens unit Gr1. When the upper limit of the conditional expression (4) is exceeded, an axial chromatic aberration is so great that it cannot be corrected easily. Reversely, when the lower limit of the conditional expression (4) is exceeded, the refractive power of the plastic lens in the first lens unit Gr1 varies with temperature variation, with the result that the focal point of the whole system varies greatly, making the optical performance of the whole system unstable against temperature variation.

The conditional expression (5) defines the refractive power of the plastic lens in the second lens unit Gr2. When the lower limit of the conditional expression (5) is exceeded, an axial chromatic aberration is so great that it cannot be corrected easily. Reversely, when the upper limit of the conditional expression (5) is exceeded, the refractive power of the plastic lens in the second lens unit Gr2 varies with temperature variation, with the result that the focal point of the whole system varies greatly, making the optical performance of the whole system unstable against temperature variation.

The conditional expression (6) defines the relation between the refractive powers of the plastic lenses in the first and second lens units Gr1 and Gr2. When the upper limit of the conditional expression (6) is exceeded, the refractive powers of the plastic lenses included in the whole zoom lens system vary with temperature variation, with the result that the focal point of the whole system shifts greatly toward the image side, destabilizing the optical performance. Reversely, when the lower limit of the conditional expression (6) is exceeded, the focal point of the whole system shifts greatly toward the object side, destabilizing the optical performance.

    0.15<φ1×T<0.26                                   (7)

where

φ1: composite refractive power of the first lens unit;

T: air gap between the negative and positive lenses included in the first lens unit.

The conditional expression (7) defines a condition that needs to be satisfied by the air gap between the positive and negative lenses in the first lens unit Gr1, and is related to correction of magnification chromatic aberration and distortion. When the lower limit of the conditional expression (7) is exceeded, the negative and positive lenses are too close to each other, and accordingly it is difficult to correct distortion. Reversely, when the upper limit of the conditional expression (7) is exceeded, the refractive power of the negative lens in the first lens unit is so strong that magnification chromatic aberration, in particular, cannot be corrected easily.

In the zoom lens systems of the embodiments 4, 5, and 8, the first lens unit Gr1 comprises, from the object-side end, a first lens L1 having either a weak refractive power or a negative refractive power, a second lens L2 having a negative power, and a third lens L3 having a positive refractive power (hereinafter referred to as the first construction). On the other hand, in the zoom lens systems of the embodiments 6 and 7, the first lens unit Gr1 comprises, from the object-side end, a first lens L1 having a negative refractive power, and a second lens L2 having a positive refractive power (hereinafter referred to as the second construction).

If one of the two constructions described above is adopted as the construction of the first lens unit Gr1 having a positive refractive power, the arrangement of refractive powers over the whole system becomes analogous to the lens arrangement for a retrofocus lens, and accordingly a sufficient lens-back is available at the wide-angle end. Thus, the outer diameter of the second lens unit Gr2 can be made smaller, and the whole system can be made more compact.

Moreover, in the first construction, if the second lens L2 has at least one aspherical surface, spherical aberration and off-axial coma aberration can be corrected properly. In particular, if an aspherical plastic lens is used as the second lens L2 having a negative refractive power, costs can be reduced, compared to an aspherical lens made of glass. In addition, since the plastic lens is disposed between first and third lenses L1 and L3 made of glass, the plastic lens, which requires careful handling to prevent scratches and other damage, can be protected against damage from outside.

Furthermore, in a two-lens-group zoom lens system, if the single lens nearest to the object side in the first lens unit Gr1 has a concave surface facing toward the object side, inclination of the image surface can be corrected, thereby giving the zoom lens system even better optical performance.

FIGS. 12A to 16I are diagrams showing aberration curves corresponding to the embodiments4 to 8, respectively. FIGS. 12A to 12C, 13A to 13C, 14A to 14C, 15A to 15C, and 16A to 16C show aberrations at the wide-angle-end focal length (at the shortest focal length), FIGS. 12D to 12F, 13D to 13F, 14D to 14F, 15D to 15F, and 16D to 16F show aberrations at the middle focal length, and FIGS. 12G to 12I, 13G to 13I, 14G to 14I, 15G to 15I, and 16G to 16I show aberrations at the telephoto-end focal length (at the longest focal length). Moreover, in spherical aberration diagrams, solid lines (d) show aberrations with d-lines, and broken lines (SC) show sine conditions. Furthermore, in astigmatism diagrams, broken lines (DM) and solid lines (DS) show astigmatisms on the meridional image surface and on the sagittal image surface, respectively.

Hereinafter, the embodiments 9 to 11 of the present invention will be described.

FIGS. 17 to 19 show the lens constructions of the embodiments 9 to 11, respectively, under the shortest-focal-position state. In each figure, the arrows m1 and m2 schematically indicate the movement of the first and second lens units Gr1 and Gr2, respectively, during zooming from the wide-angle end (W) to the telephoto end (T). The embodiments will be described below with reference to FIG. 17.

The zoom lens systems of these embodiments comprises, from the object side, a first lens unit Gr1 composed only of a positive meniscus lens L1 with its concave surface facing toward the object side, and a second lens unit Gr2 having a negative refractive power. The second lens unit Gr2 is composed of a negative meniscus lens L2 with its concave surface facing toward the object side, and a negative meniscus lens L3 with its concave surface facing toward the object side. Moreover, an aperture diaphragm A is disposed to the image side of the first lens unit Gr1.

During zooming from the wide-angle end (W) to the telephoto end (T), the first lens unit Gr1 and the aperture diaphragm A move toward the object side, and the second lens unit Gr2 also moves toward the object side in such a way that the distance between the first and second lens units Gr1 and Gr2 decreases.

In these embodiments, the first lens unit is composed of one lens, that is, it is composed of a minimum number of lens. On the other hand, the second lens unit is composed of two lenses, so that magnification chromatic aberration can be corrected properly by the second lens unit.

Alternatively, in a zoom lens system which comprises, from the object side, a first lens unit composed of one positive lens, and a second lens unit having a negative refractive power and which zooms from the wide-angle end to the telephoto end by decreasing the distance between the first and second lens units, if the zoom lens system satisfies the following conditional expression (8), axial chromatic aberration can be corrected properly.

    0.9<|φ1/φ2|<1.3                  (8)

where

φ1: composite refractive power of the first lens unit;

φ2: composite refractive power of the second lens unit.

In general, chromatic aberration in a zoom lens system is corrected within each lens unit in order to reduce effects resulting from magnification during zooming. In these embodiments, however, since the first lens unit is composed of one lens, chromatic aberration cannot be corrected within the first lens unit. For this reason, in these embodiments, chromatic aberration occurring in the first lens unit is corrected by the second lens unit, so that axial chromatic aberration of the whole system is corrected properly. More specifically, by constructing the first and second lens units so that their respective composite refractive powers will satisfy the above-mentioned conditional expression (8), axial chromatic aberration can be corrected properly. In addition, the Petzval sum can be corrected properly as well.

When the lower limit of the conditional expression (8) is exceeded, axial chromatic aberration occurring in the first lens unit cannot be corrected easily by the second lens unit. Reversely, when the upper limit of the conditional expression (8) is exceeded, the refractive power of the second lens unit is smaller relative to that of the first lens unit, with the result that the Petzval sum is so large in the positive direction over the whole zooming range that it cannot be corrected easily.

In these embodiments, the first lens unit is composed of one lens, and chromatic aberration occurring in the first lens unit is corrected by the second lens unit. For this reason, it is desirable that the following conditional expression (9) be satisfied:

    ν1>55                                                   (9)

where

ν1: Abbe number of the lenses of the first lens unit.

If the conditional expression (9) above is satisfied, chromatic aberration occurring in the first lens unit can be reduced to a minimum. A value below the lower limit specified by the conditional expression (9) indicates a high degree of dispersion, which makes chromatic aberration occurring in the first lens unit so great that it cannot be corrected easily by the second lens unit. Under this condition, chromatic aberration occurring in the first lens unit needs to be corrected within the first lens unit in some way, for example, by constructing the first lens unit with more than one lens.

Moreover, in a zoom lens system comprising a first lens unit composed of one positive lens, and a negative second lens unit composed of two lenses, it is desirable that the following conditional expression (10) be satisfied:

    -3.0<r/T<-1.7                                              (10)

where

r: object-side curvature radius of the image-side lens in the second lens unit;

T: air gap between the object-side and image-side lenses in the second lens unit.

When the upper limit of the conditional expression (10) is exceeded, the curvature radius of the object-side surface of the image-side lens is so small that a sufficient distance cannot be obtained between the object-side and image-side lenses, with the result that off-ax:ial light beams cannot be secured easily. Reversely, when the lower limit of the conditional expression (10) is exceeded, magnification chromatic aberration cannot be corrected easily.

Moreover, in a zoom lens system comprising a first lens unit composed of one positive lens, and a negative second lens unit composed of two lenses, it is desirable that the second lens unit be composed of a plastic lens having at least one aspherical surface and having almost no refractive power, and a lens having a negative refractive power. In this construction, magnification chromatic aberration and distortion occurring in the first lens unit can be corrected by the aspherical surface of the plastic lens. Furthermore, use of the plastic lens having almost no refractive power makes it possible to suppress shifting of the focal position caused by the plastic lens affected by temperature variation. Here, a lens having almost no refractive power refers to a lens whose refractive power φp satisfies the following conditional expression (11):

    0≦|φp/φw|<0.2             (11)

where

φw: refractive power of the whole system at the wide-angle end.

Hereinafter, the embodiments 9 to 11 of the present invention will be presented on Tables 12 to 14. Note that Tables 12 to 14 are described in the same manner as Tables 1 to 3.

                  TABLE 12                                                         ______________________________________                                         Embodiment 9                                                                   ______________________________________                                         f = 36.0˜46.0˜58.5                                                 Fno = 5.90˜7.53˜9.57                                               Curvature                                                                               Axial Surface Refractive                                              Radius   Distance      Index      Abbe Number                                  ______________________________________                                         r1* -48.073                                                                             d1 7.116      N1 1.51178 ν1 69.07                                  r2* -8.333                                                                              d2 1.000                                                              r3 ∞(aperture)                                                                    d3 6.361˜3.903˜2.000                                      r4* -16.840                                                                             d4 5.000      N2 1.62017 ν2 24.01                                  r5* -18.754                                                                             d5 3.400                                                              r6 -6.789                                                                               d6 3.349      N3 1.63980 ν3 34.55                                  r7 -14.252                                                                     ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.50176 × 10E-3                                                    A6 = 0.66872 × 10E-5                                                     A8 = -0.11529 × 10E-5                                                    A10 = 0.23216 × 10E-7                                                    A12 = -0.20129 × 10E-8                                                   A14 = 0.17255 × 10E-9                                                    r2                                                                             ε = 0.10000 × 1                                                  A4 = 0.39318 × 10E-6                                                     A6 = -0.20835 × 10E-5                                                    A8 = 0.31160 × 10E-6                                                     A10 = -0.15213 × 10E-7                                                   A12 = -0.51514 × 10E-9                                                   A14 = 0.46299 × 10E-10                                                   r4                                                                             ε = 0.10000 × 10                                                 A4 = 0.17577 × 10E-3                                                     A6 = 0.15346 × 10E-6                                                     A8 = 0.20743 × 10E-6                                                     A10 = -0.14592 × 10E-8                                                   A12 = -0.14056 × 10E-9                                                   A14 = 0.22878 × 10E-11                                                   A16 = 0.60774 × 10E-13                                                   r5                                                                             ε = 0.10000 × 1                                                  A4 = -0.97056 × 10E-4                                                    A6 = 0.39243 × 10E-5                                                     A8 = -0.19724 × 10E-6                                                    A10 = 0.19476 × 10E-8                                                    A12 = 0.49383 × 10E-10                                                   A14 = -0.10674 × 10E-11                                        ______________________________________                                    

                  TABLE 13                                                         ______________________________________                                         Embodiment 10                                                                  ______________________________________                                         f = 36.0˜46.0˜58.5                                                 Fno = 5.90˜7.53˜9.57                                               Curvature                                                                               Axial Surface Refractive                                              Radius   Distance      Index      Abbe Number                                  ______________________________________                                         r1* -29.846                                                                             d1 7.116      N1 1.49310 ν1 83.58                                  r2* -7.534                                                                              d2 2.000                                                              r3 ∞(aperture)                                                                    d3 5.901˜3.702˜2.000                                      r4* -15.132                                                                             d4 5.000      N2 1.62017 ν2 24.01                                  r5* -17.046                                                                             d5 3.400                                                              r6 -8.000                                                                               d6 2.000      N3 1.70154 ν3 41.15                                  r7 -18.036                                                                     ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.61174 × 10E-3                                                    A6 = 0.72822 × 10E-5                                                     A8 = -0.11517 × 10E-5                                                    A10 = 0.23230 × 10E-7                                                    A12 = -0.20122 × 10E-8                                                   A14 = 0.17255 × 10E-9                                                    r2                                                                             ε = 0.10000 × 1                                                  A4 = 0.40939 × 10E-4                                                     A6 = 0.52954 × 10E-6                                                     A8 = 0.27879 × 10E-6                                                     A10 = -0.15253 × 10E-7                                                   A12 = -0.51615 × 10E-9                                                   A14 = 0.46304 × 10E-10                                                   r4                                                                             ε = 0.10000 × 10                                                 A4 = 0.16688 × 10E-3                                                     A6 = -0.12012 × 10E-5                                                    A8 = 0.18079 × 10E-6                                                     A10 = -0.18493 × 10E-8                                                   A12 = -0.17535 × 10E-9                                                   A14 = 0.21460 × 10E-11                                                   A16 = 0.60774 × 10E-13                                                   r5                                                                             ε = 0.10000 × 1                                                  A4 = -0.71254 × 10E-4                                                    A6 = 0.39805 × 10E-5                                                     A8 = -0.19094 × 10E-6                                                    A10 = 0.19794 × 10E-8                                                    A12 = 0.49707 × 10E-10                                                   A14 = -0.99695 × 10E-11                                        ______________________________________                                    

                  TABLE 14                                                         ______________________________________                                         Embodiment 11                                                                  ______________________________________                                         f = 36.0˜46.0˜58.5                                                 Fno = 5.90˜6.99˜9.31                                               Curvature                                                                               Axial Surface Refractive                                              Radius   Distance      Index      Abbe Number                                  ______________________________________                                         r1* -48.073                                                                             d1 7.116      N1 1.49300 ν1 58.34                                  r2* -8.067                                                                              d2 1.000                                                              r3 ∞(aperture)                                                                    d3 6.492˜3.960˜2.000                                      r4* -17.119                                                                             d4 5.000      N2 1.62017 ν2 24.01                                  r5* -20.145                                                                             d5 3.400                                                              r6 -6.688                                                                               d6 3.349      N3 1.67339 ν3 29.25                                  r7 -12.853                                                                     ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.51911 × 10E-3                                                    A6 = 0.72617 × 10E-5                                                     A8 = -0.11519 × 10E-5                                                    A10 = 0.23222 × 10E-7                                                    A12 = -0.20128 × 10E-8                                                   A14 = 0.17255 × 10E-9                                                    r2                                                                             ε = 0.10000 × 1                                                  A4 = 0.20926 × 10E-4                                                     A6 = -0.17768 × 10E-5                                                    A8 = 0.32896 × 10E-6                                                     A10 = -0.15209 × 10E-7                                                   A12 = -0.51511 × 10E-9                                                   A14 = 0.46299 × 10E-10                                                   r4                                                                             ε = 0.10000 × 10                                                 A4 = 0.17544 × 10E-3                                                     A6 = 0.50044 × 10E-6                                                     A8 = 0.21106 × 10E-6                                                     A10 = -0.14237 × 10E-8                                                   A12 = -0.13854 × 10E-9                                                   A14 = 0.22923 × 10E-11                                                   A16 = 0.60774 × 10E-13                                                   r5                                                                             ε = 0.10000 × 1                                                  A4 = -0.10549 × 10E-3                                                    A6 = 0.38992 × 10E-5                                                     A8 = -0.19686 × 10E-6                                                    A10 = 0.19360 × 10E-8                                                    A12 = 0.47514 × 10E-10                                                   A14 = -0.10787 × 10E-11                                        ______________________________________                                    

In any of the embodiments 9 to 11, the second lens L2, which is the object-side lens in the second lens unit Gr2, is a plastic lens and has almost no refractive power.

FIGS. 20A to 22I are diagrams showing aberration curves corresponding to the embodiments 9 to 11, respectively. FIGS. 20A to 20C, 21A to 21C, and 22A to 22C show aberrations at the wide-angle-end focal length (at the shortest focal length), FIGS. 20D to 20F, 21D to 21F, and 22D to 22F show aberrations at the middle focal length, and FIGS. 20G to 20I, 21G to 21I, and 22G to 22I show aberrations at the telephoto-end focal length (at the longest focal length). Moreover, solid lines (d) show aberrations with d-lines, and broken lines (SC) show sine conditions. Furthermore, broken lines (DM) and solid lines (DS) show astigmatisms on the meridional image surface and on the sagittal image surface, respectively.

In Table 15 below are shown the values corresponding to the conditional expressions (8) to (11) in the embodiments 9 to 11.

                  TABLE 15                                                         ______________________________________                                                   Condi- Condi-  Condi-     Condi-                                               tional tional  tional     tional                                               Expres-                                                                               Expres- Expres-    Expres-                                              sion (8)                                                                              sion (9)                                                                               sion (10)  sion (11)                                            r/T    ν1   |φ1/φ2|                                                         |φp/φw|          ______________________________________                                         Embodiment 9                                                                               -2.00    69.67   1.19     0                                        Embodiment 10                                                                              -2.35    83.58   1.07     0                                        Embodiment 11                                                                              -1.97    58.34   1.22     -0.002                                   ______________________________________                                    

Hereinafter, the embodiments 12 to 15 of the present invention will be described.

FIGS. 23 to 25 show the lens constructions of the embodiments 12 to 14, respectively, under the shortest-focal-position state. These embodiments are also referred to as a third construction. FIG. 26 shows the lens construction of the embodiment 15 under the shortest-focal-position state. This embodiment is also referred to as a fourth construction. In FIGS. 23 to 26, the arrows m1 and m2 schematically indicate the movement of the first and second lens units Gr1 and Gr2, respectively, during zooming from the wide-angle end (W) to the telephoto end (T). First, the embodiments of the third construction will be described below with reference to FIG. 23.

A zoom lens system of the first construction comprises, from the object side, a first lens unit Gr1 having a positive refractive power, and a second lens unit Gr2 composed only of a negative meniscus lens with its convex surface facing toward the image side. The first lens unit Gr1 is composed of a negative meniscus lens L1 with its convex surface facing toward the object side, and a biconvex lens L2. Moreover, an aperture diaphragm A is disposed to the image side of the first lens unit Gr1.

A zoom lens system of the fourth construction comprises, from the object side, a first lens unit Gr1 having a positive refractive power, and a second lens unit Gr2 composed only of a negative meniscus lens with its convex surface facing toward the object side. The first lens unit Gr1 is composed of a negative meniscus lens L1 with its convex surface facing toward the object side, and a positive meniscus lens L2 with its convex surface facing toward the image side. Moreover, an aperture diaphragm A is disposed to the image side of the first lens unit Gr1.

In either of the third and fourth constructions, during zooming from the wide-angle end (W) to the telephoto end (T), the first lens unit Gr1 and the aperture diaphragm A moves toward the object side, and the second lens unit Gr2 also moves toward the object side in such a way that the distance between the first and second lens units Gr1 and Gr2 decreases.

In general, chromatic aberration in a zoom lens system is corrected within each lens unit in order to reduce effects resulting from magnification during zooming. In the zoom lenses of these embodiments, since the first lens unit is composed of two lenses, chromatic aberration occurring within the first lens unit is properly corrected. Nevertheless, since the second lens unit is composed of one lens, chromatic aberration resulting from differences in refractive power and dispersion between the lenses cannot be corrected within the second lens unit. For this reason, in these embodiments, the zoom lens system is so designed that the following conditional expressions (12) and (13) are satisfied, so that, by reducing chromatic aberration occurring in the second lens unit, aberration is corrected properly.

    N21<1.55                                                   (12)

    ν21>55                                                  (13)

where

N21: d-lines refractive coefficient of the negative lens in the second lens unit;

ν21: Abbe number of the negative lens in the second lens unit.

When the upper limit of the conditional expression (12) is exceeded, the refractive power of the negative lens in the second lens unit is too strong, making the Petzval sum so large that it cannot be corrected easily. On the other hand, satisfying the conditional expression (13) means alleviating the duty of the first lens unit to correct chromatic aberration, and thus leads to reduction of the total length of the first lens unit. When the lower limit of the conditional expression (13) is exceeded, a high degree of dispersion in the negative lens of the second lens unit makes magnification chromatic aberration occurring in the second lens unit so large that it is difficult to construct the second lens unit with one lens. Magnification chromatic aberration occurring in the second lens unit may be corrected effectively by securing a larger distance between the object-side and image-side lenses in the first lens unit. A larger distance between the object-side and image-side lenses in the first lens unit, however, makes the total length of the first lens unit longer, and accordingly is not desirable in terms of compactness.

To reduce the size of a camera in the not-in-use state where its lenses are retracted, it is necessary to reduce the thickness of every lens unit in the zoom lens system. Usually, the thickness of a lens unit refers to its thickness along the optical axis; in reality, however, the thickness at its periphery also needs to be reduced. In these embodiments, the first lens unit has, at is object-side end, a convex surface facing toward the object side. As a result, the whole system forms a nearly symmetrical form, and this helps to correct distortion properly. In addition, since the periphery (edge) of the lens is positioned closer to the image side than the lens's vertex, it is possible to reduce the size of the camera in the not-in-use state. Furthermore, if the negative lens in the second lens unit has a convex surface facing toward the image side, the periphery (edge) of this lens is positioned closer to the object side than the lens's vertex, and accordingly it is possible to reduce the size of the camera in the not-in-use state.

In terms of compactness, it is preferable that a construction of these embodiments satisfy the following conditional expression (14):

    0.35<ΣT×φ1<0.45                            (14)

where

ΣT: sum of the axial thickness of the first lens unit (that is, the distance from the object-side vertex of the object-side lens L1 to the image-side vertex of the image-side lens L2) and the axial thickness of the negative lens in the second lens unit (hereinafter referred to as the lens's total length);

φ1: composite refractive power of the first lens unit.

The conditional expression (14) above defines the ratio of the total length of the lens to the composite refractive power of the first lens unit. When the lower limit of the conditional expression (14) is exceeded, chromatic aberration occurring in the first lens unit is so great that it cannot be corrected easily. Reversely, when the upper limit of the conditional expression (14) is exceeded, the total length of the lens is so large that it impairs the compactness of the camera.

Moreover, as long as glass aspherical lenses are used, much is not expected in cost reduction; use of plastic lenses is essential for further cost reduction. In these embodiments, the first lens unit includes at least one plastic lens, and, by constructing the second lens unit with plastic lenses, costs can effectively reduced.

Furthermore, in these embodiments, it is preferable that at least one aspherical surface be included in the negative lens in the first lens unit and in the second lens unit. If the first lens unit includes at least one aspherical surface, spherical aberration occurring in the first lens unit can be corrected properly. On the other hand, if the second lens unit includes at least one aspherical surface, off-axial coma aberration and distortion can be corrected properly.

Hereinafter, the embodiments 12 to 15 of the present invention will be presented on Tables 16 to 20. Note that Tables 16 to 20 are described in the same manner as Tables 1 to 3.

                  TABLE 16                                                         ______________________________________                                         Embodiment 12                                                                  ______________________________________                                         f = 36.0˜45.9˜58.5                                                 Fno = 5.80˜6.87˜9.12                                               Curvature                                                                               Axial Surface  Refractive                                             Radius   Distance       Index     Abbe Number                                  ______________________________________                                         r1* 37.719                                                                              d1 1.950       N1 1.84506                                                                               ν1 23.66                                  r2* 22.058                                                                              d2 3.400                                                              r3 88.706                                                                               d3 2.900       N2 1.48749                                                                               ν2 70.44                                  r4 -10.240                                                                              d4 0.920                                                              r5 ∞(aperture)                                                                    d5 17.317˜13.612˜10.71                                    r6* -9.639                                                                              d6 0.900       N3 1.51178                                                                               ν3 69.07                                  r7* -40.333                                                                    ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.48194 × 10E-3                                                    A6 = 0.25659 × 10E-5                                                     A8 = -0.77243 × 10E-7                                                    A10 = 0.11717 × 10E-9                                                    A12 = 0.52725 × 10E-10                                                   A14 = -0.26407 × 10E-11                                                  r2                                                                             ε = 0.10000 × 10                                                 A4 = -0.39105 × 10E-3                                                    A6 = 0.33311 × 10E-5                                                     A8 = 0.37167 × 10E-7                                                     A10 = -0.48424 × 10E-9                                                   A12 = -0.10505 × 10E-9                                                   r6                                                                             ε = 0.10000 × 10                                                 A4 = 0.35183 × 10E-3                                                     A6 = -0.13159 × 10E-4                                                    A8 = 0.63200 × 10E-6                                                     A10 = -0.13491 × 10E-7                                                   A12 = 0.12801 × 10E-9                                                    A14 = -0.41110 × 10E-12                                                  r7                                                                             ε = 0.10000 × 10                                                 A4 = 0.13547 × 10E-3                                                     A6 = -0.28511 × 10E-6                                                    A8 = -0.37787 × 10E-8                                                    A10 = -0.70867 × 10E-10                                                  A12 = 0.90548 × 10E-12                                                   A14 = -0.20558 × 10E-14                                        ______________________________________                                    

                  TABLE 17                                                         ______________________________________                                         Embodiment 13                                                                  ______________________________________                                         f = 36.0˜45.9˜68.5                                                 Fno = 5.40˜6.89˜10.28                                              Curvature                                                                               Axial Surface  Refractive                                             Radius   Distance       Index     Abbe Number                                  ______________________________________                                         r1* 15.284                                                                              d1 2.531       N1 1.84666                                                                               ν1 23.82                                  r2* 10.517                                                                              d2 4.500                                                              r3 52.410                                                                               d3 3.410       N2 1.48749                                                                               ν2 70.44                                  r4 -10.018                                                                              d4 1.533                                                              r5 ∞(aperture)                                                                    d5 16.116˜12.426˜8.000                                    r6* -10.304                                                                             d6 0.800       N3 1.51178                                                                               ν3 69.07                                  r7* -54.368                                                                    ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.34523 × 10E-3                                                    A6 = 0.35969 × 10E-5                                                     A8 = -0.18748 × 10E-6                                                    A10 = 0.41923 × 10E-9                                                    A12 = 0.77293 × 10E-10                                                   A14 = -0.12540 × 10E-11                                                  r2                                                                             ε = 0.10000 × 10                                                 A4 = -0.31344 × 10E-3                                                    A6 = 0.34395 × 10E-6                                                     A8 = 0.36539 × 10E-7                                                     A10 = -0.46741 × 10E-9                                                   A12 = -0.10495 × 10E-9                                                   r6                                                                             ε = 0.10000 × 10                                                 A4 = 0.42752 × 10E-3                                                     A6 = -0.97585 × 10E-5                                                    A8 = 0.13792 × 10E-6                                                     A10 = 0.76947 × 10E-9                                                    A12 = -0.37263 × 10E-10                                                  A14 = 0.26329 × 10E-12                                                   r7                                                                             ε = 0.10000 × 10                                                 A4 = 0.22755 × 10E-3                                                     A6 = -0.37558 × 10E-5                                                    A8 = 0.28305 × 10E-7                                                     A10 = 0.12918 × 10E-9                                                    A12 = -0.33933 × 10E-11                                                  A14 = 0.14544 × 10E-13                                         ______________________________________                                    

                  TABLE 18                                                         ______________________________________                                         Embodiment 14                                                                  ______________________________________                                         f = 36.0˜45.9˜58.5                                                 Fno = 5.80˜6.87˜9.12                                               Curvature                                                                               Axial Surface   Refractive                                                                               Abbe                                        Radius   Distance        Index     Number                                      ______________________________________                                         r1* 28.026                                                                              d1 1.900        N1 1.62017                                                                               ν1 24.01                                 r2* 14.868                                                                              d2 3.600                                                              r3 74.988                                                                               d3 3.500        N2 1.48749                                                                               ν2 70.44                                 r4 -10.070                                                                              d4 1.533                                                              r5 ∞(aperture)                                                                    d5 17.679˜13.934˜11.000                                   r6* -l0.637                                                                             d6 1.200        N3 1.49300                                                                               ν3 58.34                                 r7* -80.645                                                                    ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.55810 × 10E-3                                                    A6 = 0.27158 × 10E-5                                                     A8 = -0.83236 × 10E-7                                                    A10 = 0.88213 × 10E-10                                                   A12 = 0.52080 × 10E-10                                                   A14 = -0.15429 × 10E-11                                                  r2                                                                             ε = 0.10000 × 10                                                 A4 = -0.42544 × 10E-3                                                    A6 = 0.35182 × 10E-5                                                     A8 = 0.38382 × 10E-7                                                     A10 = -0.48088 × 10E-9                                                   A12 = -0.10504 × 10E-9                                                   r6                                                                             ε = 0.10000 × 10                                                 A4 = 0.32218 × 10E-3                                                     A6 = -0.11562 × 10E-4                                                    A8 = 0.49764 × 10E-6                                                     A10 = -0.11956 × 10E-7                                                   A12 = 0.14016 × 10E-9                                                    A14 = -0.61299 × 10E-12                                                  r7                                                                             ε = 0.10000 × 10                                                 A4 = 0.13693 × 10E-3                                                     A6 = -0.13115 × 10E-5                                                    A8 = -0.19093 × 10E-8                                                    A10 = 0.99314 × 10E-10                                                   A12 = -0.35176 × 10E-12                                                  A14 = -0.29031 × 10E-15                                        ______________________________________                                    

                  TABLE 19                                                         ______________________________________                                         Embodiment 15                                                                  ______________________________________                                         f = 36.0˜45.9˜58.5                                                 Fno = 5.80˜6.87˜9.12                                               Curvature                                                                               Axial Surface  Refractive                                             Radius   Distance       Index     Abbe Number                                  ______________________________________                                         r1* 16.867                                                                              d1 1.600       N1 1.62017                                                                               ν1 24.01                                  r2* 9.598                                                                               d2 3.200                                                              r3 -120.440                                                                             d3 3.410       N2 1.52510                                                                               ν2 56.38                                  r4 -7.911                                                                               d4 1.533                                                              r5 ∞(aperture)                                                                    d5 14.798˜11.379˜8.700                                    r6* -11.896                                                                             d6 1.000       N3 1.52510                                                                               ν3 56.38                                  r7* -130.746                                                                   ______________________________________                                         Aspherical Coefficient                                                         ______________________________________                                                   r1                                                                             ε = 0.10000 × 10                                                 A4 = -0.78927 × 10E-3                                                    A6 = 0.11393 × 10E-5                                                     A8 = -0.14705 × 10E-6                                                    A10 = -0.10890 × 10E-9                                                   A12 = 0.47116 × 10E-10                                                   A14 = -0.19103 × 10E-10                                                  r2                                                                             ε = 0.10000 × 10                                                 A4 = -0.55901 × 10E-3                                                    A6 = 0.45074 × 10E-5                                                     A8 = 0.41561 × 10E-7                                                     A10 = -0.47538 × 10E-9                                                   A12 = -0.10504 × 10E-9                                                   r6                                                                             ε = 0.10000 × 10                                                 A4 = 0.47710 × 10E-3                                                     A6 = -0.11552 × 10E-4                                                    A8 = 0.40407 × 10E-6                                                     A10 = -0.14217 × 10E-7                                                   A12 = 0.23489 × 10E-9                                                    A14 = -0.13333 × 10E-11                                                  r7                                                                             ε = 0.10000 × 10                                                 A4 = 0.27870 × 10E-3                                                     A6 = -0.42449 × 10E-5                                                    A8 = 0.37276 × 10E-9                                                     A10 = 0.20256 × 10E-9                                                    A12 = 0.16134 × 10E-11                                                   A14 = -0.17528 × 10E-13                                        ______________________________________                                    

FIGS. 27A to 30I are diagrams showing aberration curves corresponding to the embodiments 12 to 15, respectively. FIGS. 27A to 27C, 28A to 28C, 29A to 29C, and 30A to 30C show aberrations at the wide-angle-end focal length (at the shortest focal length), FIGS. 27D to 27F, 28D to 28F, 29D to 29F, and 30D to 30F show aberrations at the middle focal length, and FIGS. 27G to 27I, 28G to 28I, 29G to 29I, and 30G to 30I show aberrations at the telephoto-end focal length (at the longest focal length). Moreover, solid lines (d) show aberrations with d-lines, and broken lines (SC) show sine conditions. Furthermore, broken lines (DM) and solid lines (DS) show astigmatisms on the meridional image surface and on the sagittal image surface, respectively.

Of the lenses used in these embodiments, the first and third lenses L1and L3 in the embodiment 14, and the first to third lenses L1 to L3 in the embodiment 15 are plastic lenses.

In Table 20 below are shown the values corresponding to the conditional expressions (12) to (14) in embodiments 12 to 15.

                  TABLE 20                                                         ______________________________________                                                    Conditional                                                                            Conditional Conditional                                                Expression                                                                             Expression  Expression                                                 (12)    (13)        (14)                                                       N21 < 1.55                                                                             ν21 > 55 ΣT × φ1                         ______________________________________                                         Embodiment 12                                                                               1.512     69.1        0.37                                        Embodiment 13                                                                               1.512     69.1        0.46                                        Embodiment 14                                                                               1.493     58.3        0.41                                        Embodiment 15                                                                               1.525     56.4        0.40                                        ______________________________________                                     

What is claimed is:
 1. A zoom lens system comprising, from an object side:a first lens unit having a positive refractive power as a whole, said first lens unit comprising, from the object side, a first lens element, a second lens element having a negative refractive power and an object side surface convex to the object side, a third lens element having a positive refractive power, said first lens unit being provided at a most object-side end of the zoom lens system, and a second lens unit having a negative refractive power, wherein, during zooming from a shortest-focal length state to a longest-focal length state, a distance between said first and second lens units is decreased, and, wherein the zoom lens system fulfills the following condition:

    0.15<φ1×T23<0.25

φ1 corresponding to the composite refractive power of the first lens unit, and T23 corresponding to an air gap between the second and the third lens element.
 2. The zoom lens system as claimed in claim 1, wherein said first lens element is made of plastic.
 3. The zoom lens system as claimed in claim 1, wherein said first lens element has at least one aspherical surface.
 4. The zoom lens system as claimed in claim 1, wherein said first lens element has a negative refractive power.
 5. The zoom lens system as claimed in claim 1, further comprising, a diaphragm is provided between said first and second lens units.
 6. The zoom lens system as claimed in claim 1, wherein said second lens unit has at least one aspherical lens surface.
 7. The zoom lens system as claimed in claim 1, wherein said second lens unit has at least one plastic lens element.
 8. A zoom lens system comprising, from an object side:a first lens unit having a positive refractive power as a whole, said first lens unit comprising, from the object side, a first lens element, a second lens element having a negative refractive power and an image side surface concave to the image side, a third lens element having a positive refractive power, said first lens unit being provided at a most object-side end of the zoom lens system, and; a second lens unit having a negative refractive power, wherein, during zooming from a shortest-focal length state to a longest-focal length state, a distance between said first and second lens units is decreased, and wherein the zoom lens system fulfils the following condition:

    0.15<φ1×T23<0.25

φ1corresponding to the composite refractive power of the first lens unit, and T23 corresponding to an air gap between the second and the third lens element.
 9. The zoom lens system as claimed in claim 8, wherein said first lens element is made of plastic.
 10. The zoom lcns system as claimed in claim 8, wherein said first lens clement has at cast one aspherical surface.
 11. The zoom lens system as claimed in claim 8, wherein said first lens element has a negative refractive power.
 12. The zoom lens system as claimed in claim 8, further comprising, a diaphragm provided between said first and second lens units.
 13. The zoom lens system as claimed in claim 8, wherein said second lens unit has at least one aspherical lens surface.
 14. The zoom lens system as claimed in claim 8, wherein said second lens unit has at least one plastic lens element. 